LE BOURGET, France—Air France Flight 447 slammed into the Atlantic Ocean, intact and belly first, at such a high speed that the 228 people aboard probably had no time to even inflate their life jackets, French investigators said Thursday in their first report into the June 1 accident.

Likening the investigation to a puzzle with missing pieces, lead investigator Alain Bouillard said that one month after the crash, “we are very far from establishing the causes of the accident.”

Problematic speed sensors on the Airbus A330-200 jet that have been the focus of intense speculation since the crash may have misled the plane’s pilots but were not a direct cause, Bouillard said, while admitting that investigators are still a long way from knowing what did precipitate the disaster.

“The investigation is a big puzzle,” said Bouillard, who is leading the probe for the French accident agency

BEA.

The plane was flying from Rio de Janeiro to Paris when it went down in a remote area of the Atlantic, 930 miles off Brazil’s mainland and far from radar coverage.